Can you explain what happens inside a closure function and how it remembers surrounding variables?

A closure in Python is a function that remembers and can use variables from its surrounding (outer) context, even after that context has finished execution.

When a nested function is defined inside another function, it captures references to variables from the outer function, not their values. These variables are stored in a special memory area called a closure, which allows the nested function to access them later.

Example:

def outer(x):
    def inner(y):
        return x + y  # x is taken from the outer function
    return inner

f = outer(10)
print(f(5))  # Outputs 15

Here, inner remembers the value of x from outer, even after outer has finished. This allows creating functions with preserved state without using global variables.